The field of infrared spectroscopy has developed to provide devices for the prompt and efficient identification of chemical substances. Many of the Fourier-transform infrared (FTIR) spectrometers systems in commercial and laboratory use today make use of a Michelson interferometer to create a time varying light wave to pass through a sample of material. Variations in the light intensity due to interference in the sample chamber are created by a moving mirror in the interferometer. Motion of the interferometer moving mirror is normally tracked by a positioning laser operating in parallel with the light source of the spectrometer, with the laser also entering the interferometer. The laser has a wavelength shorter than the light source so that the light intensity changes due to interference in the output from the interferometer gives precise positioning information as to the change in position of the moving mirror.
The operation of such a Michelson interferometer FTIR spectrometer system is thus critically dependent on the changing position, speed, and control of the moving mirror. Since the motion of the mirror is desired to be constant, and since the mirror must stop and change direction, it is normal to have a series of position detectors to detect the position of the mirror during its accelerating and constant velocity phases, so that precise information as to the location of the mirror can be utilized by the control circuitry of the FTIR. Such information is necessary to determine when to sample and digitize data. Usually the position sensors for identifying the exact location of the mirror include several sets of photodetectors and appropriate interrupting tabs, located on the moving carriage of the mirror, arranged along the track of motion of the mirror. Thus, for example, one photodetector would typically be located at the extreme position of the withdrawal of the mirror, and a second photodetector would be necessary to indicate when the start of scan position of the mirror had been reached.
One disadvantage of such a system is that the positioning of the mirror is then fixed by the hardware of the spectrometer. In other words, if a single start of scan photodetector is hard-wired into the system, the scanning always begins at that location. While this is suitable for many applications, in some particularized applications it is desired to advance or retard the position of the start of scan, or to delay the period in which data is taken for some particular purposes. By operating the moving mirror start of scan position under software control, with an adjustable position control for the taking of data, such an objective can be realized.